In a commonly used ion mobility spectrometer, ions are generated in an ion source and then introduced into a drift region of the spectrometer by a gating grid over a short period of time. An axial electric field pulls the ions of these ion pulses through a stationary drift gas in the drift region, their velocity being determined by their “mobility”. At the end of the drift region, the incident ion current is measured at an ion detector, digitized and stored as a “mobility spectrum” in the form of a sequence of digitized ion current values. This series of ion current values is usually converted into a peak list, which contains only the drift times of the peaks and their peak heights.
In the ion source of an ion mobility spectrometer, several ion species such as monomer ions, dimer ions, dissociative ions and associative ions are usually formed from the molecules of one substance. Some substances even form ions of greater complexity. The number of ions formed can be further increased by the addition of doping agents. The ions of the substances are usually formed by so-called “chemical ionization at atmospheric pressure” (APCI) in reactions with reactant ions, usually by protonation or deprotonation of the substance molecules or by electron transfer or ion attachment. The reactant ions are usually generated in the ion source by irradiating the carrier gas with ionizing radiation. The intensity ratios of the individual ion species with respect to each other depend on the concentration of the analyte molecules and on their affinity for the different types of charge transfer in the ion source.
The mobility of the ions can be used for the identification of a substance, the mobility of the main signal, usually the monomer ion, being the one which is often used. The identification can be confirmed by the mobility of a secondary signal, usually of the dimer ion or a dissociation ion. Usually, both positive and negative ions can be measured in mobility spectrometers by switching over the voltage. For some substances, both positive and negative ions are formed; the mobilities of the ions of the other polarity can then be used to confirm the identity.
The mobility values of the relevant signals of known harmful substances, or even their full mobility spectra, are stored in corresponding data collections, which are termed libraries, or more precisely reference libraries. The diffusion broadening of the mobility signals limits the mobility resolution, while the low stability of the pressure and temperature sensors, whose measurements are required to calculate normalized mobilities, limit the accuracy of the mobility determination. Consequently, comparisons with mobility values in libraries must be performed with relatively large tolerances, which amount to at least around one percent of the mobility value; the small number of peaks and the low mobility accuracy mean that reliable identification is not always guaranteed.
This method of identification is quite successful if the types of harmful substance which can occur are severely limited in number and rarely suffer from interferences with other substances. This is the case with the analysis of military warfare agents, for example. In other application fields, such as the testing of suitcases for adherent traces of explosives, however, this type of identification is not sufficient because a large number of substances can interfere with the measurement.
As an example, FIGS. 7 and 8 show the series of spectra of a sample named “SPICE” and the explosive “TNT”. If one compares only two characteristic mobility spectra from this, as is shown in FIG. 9, they are almost identical, although the series of spectra as a whole look very different.
The detection of explosives on site (e.g., on suitcases at airports) is therefore a particular challenge. The usual procedure is to take swab samples from the outside of the suitcases and vaporize them at the entrance of the ion source of the mobility spectrometer. The problem is that the measurements are frequently disturbed by other substances in the suitcases, such as essential oils from perfumes, talcum powders, soaps or spices, which often cause a false alarm because they generate ions of the same mobility as the target substances, as is depicted in FIG. 9. A large number of false alarms lead to a rejection of the method.
The identification can be improved if rather than just individual ion mobility spectra complete series of spectra are acquired, the analytes introduced as vapor pulses, whereby the concentration of the analytes in the ion source passes through an ascending and descending curve. The measured data of a series of spectra are arranged along two axis, a sample axis (related to the sequence of spectra) and a mobility axis (related to a single ion mobility spectrum, also often termed as drift time axis). As can been seen in FIGS. 7 and 8, the sample axis is a “slow axis” with a range of tens of seconds, whereas the mobility axis is a “fast axis” with a range of tens of milliseconds. A method of this type is disclosed in U.S. Published Application No. 20110042559 (S. Klepel). Since the formation of the different ion species of a substance under analysis depends on their concentration in the ion source, a series of ion mobility spectra is acquired, in which the spectra change greatly from another along the sample axis. The information content of such a series of spectra is by far greater than that of an individual mobility spectrum, but the evaluation of the information, which is done by similarity comparisons with series of reference spectra from a library, is much more difficult.
U.S. Pat. No. 7,541,577 also discloses an identification method which is based on series of spectra and is also particularly suitable for explosives. A so-called “peak shifting” of the mobility signals in a series of mobility spectra, i.e., variabilities of the mobility of the ions formed, is used for an identification. The variabilities are attributed to nonlinear and concentration-dependent behavior in the presence of labeling substances (“taggants”), which must be present in the explosives.
When the terms “spectra”, “mobility spectra” and “series of spectra” are used below, they refer to “ion mobility spectra” or “series of ion mobility spectra” unless specifically indicated otherwise. The spectra or series of spectra can have been measured using a drift region ion mobility spectrometer described above, and also with other types of mobility spectrometer.
There is a need to increase the certainty of identification for substances under analysis, particularly explosives, by evaluating series of ion mobility spectra. At the same time, the extent of the information processing is to be kept to an acceptably low level so that it is also possible to carry out identifications with relatively a simple instrument and a processor/computer in a relatively rapid sequence.